Illuminated Light Box With Light-Emitting Diodes

ABSTRACT

The invention relates to a light box, intended especially for the backlighting of liquid crystal displays. The box includes a lightguide in the form of a thin plate having two opposed main faces and at least two edges, the lightguide having a light-diffusing optical structure on one of the faces. Light-emitting diodes are placed linearly along at least one of the edges of the lightguide, the light emitted by the light-emitting diodes illuminates the lightguide via the edge, and is being diffused by that face of the lightguide having the optical structure. The light-emitting diodes placed along each of the edges of the lightguide are organized in at least two rows and in such a way that a light-emitting diode in one row supplied by an electric power supply (S 1  to S 8 ) is immediately followed by a light-emitting diode in the other row supplied by another electric power supply.

The invention relates to a light box, intended especially for thebacklighting of liquid crystal displays.

The backlighting of liquid crystal displays for avionics is usuallyaccomplished by light boxes containing fluorescent tubes. These displaysproduce luminosities of the order of 1000 Cd/m² necessary for daytimevision, sometimes in bright sunlight. Furthermore, the illuminationoutput by the light box must be able to be adjusted so as to greatlyreduce its luminosity during night flights, for example to levels of theorder of 0.1 Cd/m².

FIGS. 1 a, 1 b and 1 c show various views of a light box of the priorart based on fluorescent tubes.

FIG. 1 a shows a sectional view of a light box of the prior art, basedon fluorescent tubes. The box in FIG. 1 a, of rectangular shape,comprises a case 10 having a bottom 12 with an opening 14 that faces thebottom 12, for backlighting a liquid crystal display 16.

The bottom 12 of the case 10 has a reflector 20 and an array offluorescent tubes 22 parallel to the bottom of the case and, across theopening 14, a light diffuser 24. The reflector 20 is of known shape sothat the light rays r emitted by the fluorescent tubes are directed ontothe light diffuser 24, uniformly illuminating the liquid crystaldisplay.

The light box further includes a waveguide 30 in the form of a platehaving two parallel faces 32, 34 and edges 36. The face 32 turned towardthe opening 14 of the box includes a diffusing structure 38 fordiffusing the light generated by a lateral fluorescent tube 40illuminating one of the edges of the lightguide.

FIG. 1 b shows a top view of the box of FIG. 1 a, showing thearrangement of the lateral fluorescent tube 40 on the edge 36 of thelightguide 30 having the diffusing structure 38 on the face 32 turnedtoward the liquid crystal display.

FIG. 1 c is a partial view of one edge of the lightguide, illuminated bythe lateral fluorescent tube 40, showing the path of the light rays Idiffused by the face 32 of the lightguide having the diffusing structure38.

When the ambient light is strong, for example, during the day, the arrayof fluorescent tubes 22 is turned on, producing a high level ofillumination for the liquid crystal display. At night, the illuminationmust be much lower. The array of fluorescent tubes is therefore turnedoff and only the single lateral fluorescent tube 40 is turned on,creating a low level of light diffused by the lightguide (or waveguide)30 through the diffuser 24 toward the liquid crystal display 16.

Furthermore, the range of variation of the light intensity offered bythe light box may be extended by controlling the power supplied to thetubes. For this purpose, the supply voltage for the fluorescent tubes isin the form of rectangular pulses having a frequency ranging from a fewtens to a few hundred hertz. The power supplied to the tubes may beadjusted by varying the duty cycle of the rectangular pulses.

The luminosity performance of light boxes based on fluorescent tubes issatisfactory for illuminating liquid crystal displays intended inparticular for avionics. However, such boxes have a large volume andrequire regular maintenance owing to the lifetime of the fluorescenttubes.

It is known today to use light-emitting diodes or LEDs instead offluorescent tubes for producing light boxes. The progress made inlight-emitting diodes allows light-emitting diode light boxes to beproduced that are less bulky than fluorescent tube light boxes and at alower cost, whilst approaching the efficiencies and luminous intensitiesobtained with fluorescent tubes.

FIG. 2 a shows a device for illuminating a light guide 40 comprisinglight-emitting diodes.

The device of FIG. 2 a comprises two rows Ra1, Rb2 of LEDs Lnm wired toa printed circuit 42. Each row Ra1, Ra2 has seven LEDs connected inseries (n being the number of the row, 1 or 2, and m the number of theLED in each row, 1 to 7). The rows of LEDs are aligned, one afteranother, along one of the edges B1 of the lightguide 40 in the form of athin plate, having two opposed main faces and four edges B1, B2, B3, B4which delimit the plate.

As in the light box shown in FIG. 1 a, the lightguide 40 has, on one ofthe faces 44, an optical structure 50 for diffusing the lightpropagating in the lightguide.

The lightguide 40 transmits the light generated by the rows Ra1, Ra2 oflight-emitting diodes L11, . . . L17 and L21, . . . L27, applied to itsedge B1, which, by propagating in the lightguide, is diffused by itsoptical structure 50 uniformly over the entire face 44 of thelightguide, as already described above.

Each row Ra1, Ra2 of light-emitting diodes is connected to an electricpower supply S1, for row Ra1, and to electric power supply S2, for rowRa2, respectively.

FIG. 2 b shows the circuit diagram for connecting the rows of diodes Ra1and Ra2, of FIG. 2 a, to their respective power supplies S1 and S2.

The light boxes of the prior art shown in FIGS. 2 a and 2 b having alight source based on light-emitting diodes have however the drawback,in the event of a diode or a power supply failing, of causing a loss ofuniformity of the luminosity on the surface to be illuminated.

This is because the result of one of the power supplies S1, S2 failingor one of the diodes of a row of light-emitting diodes being turned offis a loss of illumination on a half-edge (B1) of the box's lightguideand a variation in the luminosity according to the illuminated portionof the display. A variation in the luminosity of the light box, andconsequently of the liquid-crystal display that it illuminates, mayimpair the legibility of instruments on board aircraft. This may be verytroublesome for the pilot.

To alleviate the drawbacks of the LED light boxes of the prior art, theinvention proposes a light box, intended especially for the backlightingof liquid crystal displays, the box comprising:

-   -   a lightguide in the form of a thin plate having two opposed main        faces and at least two edges, the lightguide having a        light-diffusing optical structure on one of the faces;    -   light-emitting diodes placed linearly along at least one of the        edges of the lightguide, the light emitted by the light-emitting        diodes which illuminates the lightguide via the edge being        diffused by that face of the lightguide having the optical        structure;

characterized in that the light-emitting diodes placed along each of theedges of the lightguide are organized in at least two rows and in such away that a light-emitting diode in one row supplied by an electric powersupply is immediately followed by a light-emitting diode in the otherrow supplied by another electric power supply.

In a first variant, the light box according to the invention includes atleast one photodetector delivering a luminous intensity signalcorresponding to the light emitted by the box. The luminous intensitydata output by the photodetector is transmitted to a device forcontrolling or servocontrolling the luminance of the light box.

In a second variant of the light box according to the invention, theLEDs of at least one of the branches (or rows) emits a light spectrumdifferent than that of the other branches.

In a third variant, the light box according to the invention includesthree photodetectors provided with color filters for capturing the lightradiation of the light box in the red, green and blue colors so as toproduce signals for chromatically balancing the rows of light-emittingdiodes.

In a preferred embodiment, the light box according to the inventionincludes a lightguide of rectangular shape having four edges and eightrows of light-emitting diodes, one pair of diode rows illuminating arespective edge of the lightguide.

A main object of this invention is to minimize the variation inuniformity of the illumination provided by a light box in the event ofone or more rows of LEDs or power supplies for the LEDs failing.

Another object of the invention is to maintain sufficient luminosity forilluminating the liquid crystal display, especially in the event of oneor more rows of light-emitting diodes of the light box failing.

The invention will be more clearly understood with the aid of exemplaryembodiments of light boxes according to the invention, with reference tothe appended figures in which:

FIGS. 1 a, 1 b and 1 c, already described, show various views of a lightbox according to the prior art, based on fluorescent tubes;

FIG. 2 a shows a device for illuminating a lightguide, comprisinglight-emitting diodes;

FIG. 2 b shows the circuit diagram for connecting the rows of diodes Ra1and Ra2, of FIG. 2 a, to their respective power supplies S1 and S2;

FIGS. 3 a and 3 b show a sectional view and a top view of a light boxaccording to the invention, respectively;

FIG. 3 c shows the circuit diagrams for connecting the LEDs of the lightbox of FIG. 3 b;

FIG. 4 shows a first variant of the light box according to theinvention;

FIG. 5 shows a simplified diagram representing the servo control of alight box having the same structure as that described in FIG. 4, whichincludes a photoelectric cell;

FIG. 6 shows a simplified diagram representing the servocontrol of thelight emitted by a light box according to the invention, which includesthree photoelectric sensors;

FIGS. 7 a, 7 b and 7 c show partial views of embodiments of light boxesaccording to the invention, compatible with night vision; and

FIG. 8 shows, by way of example, a light box comprising a stack of twostages of LEDs and lightguides.

FIGS. 3 a and 3 b show a sectional view and a top view of a light boxaccording to the invention, respectively. The light box of FIGS. 3 a and3 b comprises a case 50 having a bottom 52 and an opening 54 facing thebottom 52 of the case, for the backlighting of a liquid crystal display56; on the bottom side of the case 50, a printed circuit 60 parallel tothe bottom of the case; and a lightguide 62 having, on one of the faces64 turned toward the bottom 52 of the case, an optical structure 70 fordiffusing the light propagating in the lightguide.

The printed circuit 60 has, on one of its faces 72, on the lightguideside, a reflector 74 for reflecting, onto the opening 54 (rays rf), thelight (rays rd) diffused by the optical structure 70 of the lightguide.

The light box of FIG. 3 a further includes, on the side of the opening54 of the box, a diffuser 75 in the form of a plate parallel to thebottom of the case and covering the entire box.

The printed circuit of the light box of FIG. 3, according to theinvention, comprises eight rows Rb1, Rb2 . . . Rb8, of seven LEDs Lnm inseries, aligned in pairs on each of the edges B1, B2, B3, B4 of thelightguide 60, respectively rows Rb1 and Rb2 illuminating the edge B1,rows Rb3 and Rb4 illuminating the edge B2, rows Rb5 and Rb6 illuminatingthe edge B3 and rows Rb7 and illuminating the edge B4.

Each of the LEDs is denoted by Lnm, n being the number of the row from 1to 8, and m the number of the LED in each row, 1 to 7 in this exemplaryembodiment, i.e. L11, L12, . . . L17 for row R1; L21, L22, . . . L27 forrow R2 and so on up to the LEDs L81, L82, . . . L87 for row R8.

Each of the rows Rb1, Rb2, . . . R8 of the LEDs is supplied by anindependent power supply S1, S2, . . . S8 respectively and according toa main feature of the invention, for a given edge of the lightguide, alight-emitting diode Lnm in a row Rbn supplied by an electric powersupply is immediately followed by a light-emitting diode L(n+1)m in theother row Rb(n+1) supplied by another electric power supply. Thus, forexample, the LEDs in rows Rb1 and Rb2 are interleaved in such a way thatthe LED L11 in row Rb1 is immediately followed by the LED L21 in rowRb2, then the latter by the LED L12 in row Rb1 and so on until the lastLED L28 in row R2 terminating the illumination of the edge B1 of thelightguide. The other edges B2, B3 and B4 have the LEDs of the otherbranches, Rb3, Rb4; Rb5, Rb6; and Rb7, Rb8, respectively, in the sameconfiguration.

FIG. 3 c shows the circuit diagrams for connecting the LEDs Lnm of thelight box of FIG. 3 b to the respective power supplies S1 to S8.

When the size of the light box is large, the edge of the lightguide mayhave more than one row of aligned diodes.

FIG. 4 shows a first variant of the light box of FIG. 3 a according tothe invention.

The light box of FIG. 4 comprises the case 50 having a bottom 52 and anopening 54 facing the bottom 52 for the backlighting of the liquidcrystal display 56 and, on the bottom side of the case 50, a printedcircuit 90 parallel to the bottom of the case and pierced by a centralhole 92, the lightguide 62 having, on one of the faces 64 turned towardthe bottom 52 of the case, an optical structure 70 for diffusing thelight propagating in the lightguide.

The printed circuit 90 is coated, on one of its faces 72, on the sidefacing the lightguide 62, with a translucent white film 94 and with adiffusing opaque coating 96 having a hole 98 in the coating coaxial withthe axis of the central hole 92 of the printed circuit 90. A diffuser 75is on top of the entire lightguide.

As in the structure of the light box shown in FIGS. 3 a and 3 b, theprinted circuit 90 comprises eight rows Rb1, Rb2, . . . Rb8, of sevenLEDs Lnm in series, aligned on each of the edges B1, B2, B3, B4 of thelightguide 62, respectively, each row being supplied by its respectivepower supply S1 to S8.

The light box of the embodiments shown in FIGS. 3 a, 3 b and 4 measures2.3 inches by 2.3 inches, the waveguide having a thickness of 1 mm. Thecentral hole 92 in the printed circuit 90 of FIG. 4 has a diameter of 3mm.

The light box of FIG. 4 further includes a printed circuit 104 having aphotoelectric sensor 106 receiving the light (rays cp) diffused by thediffusing optical structure 70 of the lightguide through the hole 98 inthe opaque coating and the central hole 92 in the printed circuit.

In this variant of FIG. 4, the photoelectric sensor (or photodetector)delivers electrical data Up as a function of the luminous intensity ofthe light box illumination. This data Up may be used to detect a failureor a drop in illumination of the light box, but also to servocontrol theluminous intensity delivered by the light box.

FIG. 5 shows a simplified diagram representing such servocontrol of alight box 110 having the same structure as that described in FIG. 4,which includes the photoelectric sensor 106.

The electrical data Up output by the photoelectric sensor 106 istransmitted, after being digitized, to a microprocessor mP 112 connectedto a CdS control device 114 for controlling the power delivered by thesupplies S1 to S8 for rows Rb1 to Rb8 of the LEDs of the light box. Thedata Up output by the photoelectric sensor is used by the microprocessor112 in order to act on the CdS control device 114 so as to maintain theluminous intensity in the light box within a specified range of possiblevariations in illumination levels output by the LEDs. For example, therows of LEDs of the light box are supplied with a periodic signal in theform of rectangular pulses, the illumination output by thelight-emitting diodes being controlled by varying the duty cycle of therectangular pulses supplying the LEDs, as described above.

In a variant of the light box according to the invention, at least onerow of diodes emits a light spectrum different than that emitted by theother rows.

In one exemplary embodiment, a light box according to the invention,having the same structure as that of the embodiment shown in FIG. 4,comprises two rows Rb1 and Rb4 of LEDs emitting a color spectrumdifferent than the other rows Rb2, Rb3, Rb5 to Rb8 of LEDs. For example,the two rows Rb1 and Rb4 emit a red color in the vicinity of 615 nm.

FIG. 6 shows a simplified diagram representing the servocontrol of thelight emitted by a light box 119 including three photoelectric sensors120, 121, 122 each having a respective red filter 130, green filter 131and blue filter 132. The sensors receive, via the hole 98 in the coatingand the hole 92 in the printed circuit of the light box, the lightdiffused by the lightguide.

The electrical data Upr, Upg, Upb output by each of the threephotodetectors corresponding to the respective red, green and bluelevels of the spectrum emitted by the light box are sent, after beingdigitized, to the input of a microprocessor mP 134 connected to a CdScontrol device 136 for controlling the power delivered by the suppliesS1 to S8 for rows Rb1 to Rb8 of the LEDs of the light box of each row ofLEDs, certain rows emitting a color spectrum different than the others.

The microprocessor mP is configured by a memory M 138 for driving thevarious rows of LEDs in such a way that the spectrum of the light outputby the box is as close as possible to the desired color, for example thecolor white, and to the specified luminous intensity.

In a variant of the embodiments shown in FIGS. 3 a, 3 b et seq., thelight box according to the invention is compatible with night vision.

Light boxes, especially for military craft or aircraft, must respectcertain radiation characteristics in the near-infrared. The LEDs used inthe light boxes radiate in the near-infrared and must, for this purpose,be filtered using optical filters.

FIGS. 7 a, 7 b and 7 c show partial views of embodiments of light boxesaccording to the invention that are compatible with night vision.

In a first embodiment, the light box of FIG. 7 a has, on the same sideas the opening 54 of the box, an optical filter 150 in the form of aplate parallel to the bottom of the case, covering the entire box, andthus filtering all of the light radiation emitted by the box. The liquidcrystal display receives filtered light radiation, the near-infraredspectrum having been filtered out.

In a second embodiment shown in FIG. 7 b, the rows of LEDs themselvesare covered by a near-infrared optical filter 152, for example in theform of a mask over the entire length of the branches of LEDs. Anothersolution could consist in molding the rows of LEDs with a material thatfilters out the near-infrared radiation and having, for example, theform of a mask 152.

In a third embodiment shown in FIG. 7 c, the LEDs are wired to theprinted circuit 90 so as to emit light radiation perpendicular to thefaces of the lightguide 62, the rows of diodes being encased in a smallcase 156, of the length of the rows, said small case being closed in itsupper part by a longitudinal near-infrared optical filter 158. A mirror160, along the entire length of the branches and inclined at 45° to thefaces of the lightguide, reflects the filtered light emitted by the rowsof LEDs onto the respective edges B1, B2, B3 and B4 of the lightguide.

The light box according to the invention may be combined with a liquidcrystal matrix having pixels arranged in a “quad” or four-pixelstructure, i.e. with red, green, blue and white pixels.

If a higher illumination intensity than that emitted by a single lightbox is required, the light box includes a stack of light-emitting diodebranches and waveguides associated with each set of branches in any oneplane. FIG. 8 shows, by way of example, a light box comprising a stackof two stages of LEDs and lightguides.

A first stage E1 has first branches Rb1 to Rb8 of LEDs wired to a firstprinted circuit 170 having a reflector 172 and its first associatedlightguide 174 and a second stage E2 having second branches Rb′1 to Rb′8wired to a second printed circuit 176, which is open over the entiresurface area of the first lightguide 174 so as to let through all of thelight from the two stages that is reflected by the reflector 172 of thefirst stage E1 and a second lightguide 178 associated with the secondbranches Rb′1 to Rb′8.

The various light boxes according to the invention ensure that thedisplays for which they are intended are more available and more easilyreadable. Furthermore, the possibility of servocontrolling theillumination ensures that the luminous intensity and colorimetrycharacteristics of the emitted light are maintained over time.

1. A light box, intended especially for the backlighting of liquidcrystal displays, the box comprising: a lightguide in the form of a thinplate having two opposed main faces and at least two edges, thelightguide having a light-diffusing optical structure on one of thefaces; light-emitting diodes placed linearly along at least one of theedges of the lightguide, the light emitted by the light-emitting diodeswhich illuminates the lightguide via the edge being diffused by thatface of the lightguide having the optical structure; wherein thelight-emitting diodes placed along each of the edges of the lightguideare organized in at least two rows and in such a way that alight-emitting diode in one row supplied by an electric power supply isimmediately followed by a light-emitting diode in the other row suppliedby another electric power supply.
 2. The light box as claimed in claim1, comprising at least one photodetector delivering a luminous intensitysignal corresponding to the light emitted by the box.
 3. The light boxas claimed in claim 2, wherein the luminous intensity data output by thephotodetector is transmitted to a device for controlling orservocontrolling the luminance of the light box.
 4. The light box asclaimed in claim 1, wherein the LEDs of at least one of the branches (orrows) emits a light spectrum different than that of the other branches.5. The light box as claimed in claim 4, comprising three photodetectorsprovided with color filters, respectively, for capturing the lightradiation of the light box in the red, green and blue colors so as toproduce signals for chromatically balancing the rows of light-emittingdiodes.
 6. The light box as claimed in claim 4, comprising a lightguideof rectangular shape having four edges and eight rows of light-emittingdiodes, one pair of diode rows illuminating a respective edge of thelightguide.
 7. The light box as claimed in, claim 1, comprising: a casehaving a bottom and an opening facing the bottom of the case, inparticular for the backlighting of a liquid crystal display; on thebottom side of the case, a printed circuit parallel to the bottom of thecase; and a lightguide having, on one of the faces turned toward thebottom of the case, an optical structure for diffusing the lightpropagating in the lightguide, the printed circuit having, on one of itsfaces, on the lightguide side, a reflector for reflecting, onto theopenings, the light diffused by the optical structure of the lightguide,the printed circuit of the light box comprising eight rows Rb1, Rb2, . .. Rb8, of seven LEDs Lnm in series, n being the number of the row from 1to 8, and m being the number of the LED in each row, the rows beingaligned in pairs on each of the edges B1, B2, B3, B4 of the lightguide,respectively, rows Rb1 and Rb2 illuminating the edge B1, rows Rb3 andRb4 illuminating the edge B2, rows Rb5 and Rb6 illuminating the edge B3and rows Rb7 and Rb8 illuminating the edge B4.
 8. The light box asclaimed in claim 7, wherein each of the rows Rb1, Rb2, . . . Rb8 of theLEDs is supplied by an independent power supply S1, S2, . . . S8respectively and in that, for a given edge of the lightguide, alight-emitting diode Lnm in a row Rbn supplied by an electric powersupply is immediately followed by a light-emitting diode L(n+1)m in theother row Rb(n+1) supplied by another electric power supply; thus, theLEDs in rows Rb1 and Rb2 are interleaved in such a way that the LED L11in row Rb1 is immediately followed by the LED L21 in row Rb2, then thelatter by the LED L12 in row Rb1 and so on until the last LED L28 in rowR2 terminating the illumination of the edge B1 of the lightguide, theother edges B2, B3 and B4 having the LEDs of the other branches, Rb3,Rb4; Rb5, Rb6; and Rb7, Rb8, respectively, in the same configuration. 9.The light box as claimed in wherein it includes, on the bottom side ofthe case, a printed circuit parallel to the bottom of the case andpierced by a central hole, the lightguide having, on one of the facesturned toward the bottom of the case, an optical structure for diffusingthe light propagating in the lightguide, the printed circuit beingcoated, on one of its faces, on the side facing the lightguide, with atranslucent white film and with a diffusing opaque coating having a holein the coating coaxial with the axis of the central hole of the printedcircuit.
 10. The light box as claimed in claim 7, comprising a printedcircuit having a photoelectric sensor receiving the light diffused bythe diffusing optical structure of the lightguide through the hole inthe opaque coating and the central hole in the printed circuit, thephotoelectric sensor delivering electrical data Up as a function of theluminous intensity of the light box illumination.
 11. The light box asclaimed in claim 10, wherein the electrical data Up output by thephotoelectric sensor is transmitted, after being digitized, to amicroprocessor connected to a control device for controlling the powerdelivered by the supplies S1 to S8 for rows Rb1 to Rb8 of the LEDs ofthe light box, the data Up output by the photoelectric sensor being usedby the microprocessor in order to act on the control device so as tomaintain the luminous intensity in the light box within a specifiedrange of possible illumination levels output by the LEDs.
 12. The lightbox as claimed in claim 1, wherein, when the rows of LEDs are suppliedwith a periodic signal in the form of rectangular pulses, theillumination output by the light-emitting diodes is controlled byvarying the duty cycle of the rectangular pulses.
 13. The light box asclaimed in claim 7, comprising three photoelectric sensors each having arespective red filter, green filter nd blue filter, the sensorsreceiving, via the hole in the coating and the hole n the printedcircuit of the light box, the light diffused by the lightguide.
 14. Thelight box as claimed in claim 12, wherein the electrical data output byeach of the three photodetectors corresponding to the respective red,green and blue levels of the spectrum emitted by the light box is sent,after being digitized, to the input of a microprocessor connected to acontrol device for controlling the power delivered by the supplies S1 toS8 for rows Rb1 to Rb8 of the LEDs of the light box of each row of LEDsemitting a different color spectrum, the microprocessor being configuredby a memory for driving the various rows of LEDs in such a way that thespectrum of the light output by the box is as close as possible to thedesired color, for example the color white, and to the desired luminousintensity.
 15. The light box as claimed in claim 1, wherein the rows ofLEDs themselves are covered by a near-infrared optical filter, in theform of a mask, over the entire length of the branches of LEDs.
 16. Thelight box as claimed in claim 1, wherein the rows of LEDs are moldedwith a material that filters out near-infrared radiation.
 17. The lightbox as claimed in claim 1, wherein the LEDs are wired to the printedcircuit so as to emit light radiation perpendicular to the faces of thelightguide, the rows of diodes being encased in a small case of thelength of the rows, said small case being closed in its upper part by alongitudinal near-infrared optical filter, a mirror along the entirelength of the branches and inclined at 45° to the faces of thelightguide, reflecting the filtered light emitted by the rows of LEDsonto the respective edges B1, B2, B3 and B4 of the lightguide.
 18. Thelight box as claimed in claim 1, wherein comprising a stack oflight-emitting diode branches and waveguides associated with each set ofbranches in any one plane.
 19. The light box as claimed in claim 18,comprising a first stage E1 having first branches Rb1 to Rb8 of LEDswired to a first printed circuit having a reflector and its firstassociated lightguide and a second stage E2 having second branches Rb′1to Rb′8 wired to a second printed circuit, which is open over the entiresurface area of the first lightguide so as to let through all of thelight from the two stages that is reflected by the reflector of thefirst stage E1 and a second lightguide associated with the secondbranches Rb′1 to Rb′8.